As is well known in the gas turbine engine field of technology, it is desirable to operate the fan at the highest tip speed possible in order to maximize the amount of air flowing through the fan to increase thrust and to enhance engine operating performance. Many advancements have occurred over the past several years for the purpose of enhancing engine operating performances and included amongst these are light weight blades, fabrication of the rotor by integrating the disk and blades, fabricating hollow blades, and fabricating the blades from plastic and/or composite materials or a combination of this material with structural and protection parts made from metal and the like. Equally well known to the skilled artisan is the fact that exceedingly high loads are transmitted from the blades to the disks which has been a constant concern to the aircraft engine designer. Axiomatically, the lower the hub-tip ratio of the blades at a given tip diameter, the smaller the diameter of the disk and hence, the load problem becomes exacerbated to a great extent. In other words by lowering the hub-tip ratio, the blading for these types of high speed fans are then supported by a disk whose diameter is reduced which places an increased burden on this smaller disk since it must be capable of withstanding substantially large loads. What is obvious is that the small diameter of the disk leaves insufficient material in the disk to accommodate a conventional blade attachment that will be capable of carrying these large blade loads.
While in some instances, integrally bladed rotors (IBR) have been utilized in advanced gas turbine engine compressors, these IBR's are not as attractive as the attachable blades when consideration is given to life cycle cost, and maintainability.
An example of a blade attachment design for the low pressure compressor or fan of a gas turbine engine is exemplified in U.S. Pat. No. 5,022,822 granted to S. M. Sincere on Jun. 11, 1991 and assigned to United Technologies Corporation, the assignee common to this patent application. As noted therein the rim and tongue and groove of the disk are designed to minimize the dead load of the disk and reduce disk size and hence, weight of the engine. Attachment schemes of this type, of necessity, must be lighter than current technology provides. Even the hollow fan blades of known technology are not sufficiently light in weight to allow the use of these attachment schemes. As is apparent from the structure in the U.S. Pat. No. 5,022,822 patent, supra, the tongue and groove is complex, requiring expensive and non-conventional machining processes imposing practical constraints when made in a production environment.
We have found that we can obviate the problems associated with the low hub-tip ratio fan blade by providing an attachment scheme that joins the airfoils to the rotor hub through sloped dovetail attachments and retain the blades axially by a segmented retainer plate located at the aft end of the attachment. A simplified blade support for radially supporting the blades under static conditions that can accommodate weights for rotor balancing for use in this configuration is contemplated.